The degradation in the charge transfer efficiency (CTE) performance of CCDs employed
on space missions is a serious concern for the Wide Field Camera 3 (WFC3) project.
The GSFC Detector Characterization Laboratory (DCL) has irradiated two large
format Marconi CCD44 devices as part of a program designed to quantify the expected
changes in performance of the WFC3 flight devices during the mission lifetime.
Both devices performed extremely well prior to irradiation. The charge transfer
efficiency (CTE) was measured using 55Fe, resulting in a parallel CTE of .999997 ± 1.0x10
-6, and a serial CTE of .999998 ± 1.0x10-6. The readout noise was
measured at a very low 2.1-2.3 e- rms. Dark current was well within
the project performance requirements of less than 20 electrons/pixel/hour. The
post-radiation results showed normal deterioration of the CTE in both devices;
the parallel CTE of .999962 ± 5.0x10-6, and a serial CTE of .999992
± 5.0x10-6 are well within the expected range. The readout noise did
not change, and dark current was again well within project requirements.

Introduction

The WFC3 is a new imaging instrument which will be deployed aboard the Hubble
Space Telescope during Servicing Mission 4. The instrument is designed with two
channels: a Near-UV/Visible (UVIS) channel covering the 0.2 to 1.0 mm wavelength
range, and a Near-IR channel extending from 0.85 mm to a long wavelength cutoff
of 1.7 mm. The UVIS channel will acquire images with a high sensitivity and with
a large (160 arcsec) FOV.

Throughout its lifetime the instrument will be subject to bombardment by high
energy particles. It is well known that the damage caused by these particles
degrades the charge transfer efficiency CTE of CCDs, and it is important to
understand how their performance will be affected. The DCL has radiation tested
two engineering grade, large format Marconi CCD44 devices (CCD44V1 and
CCD44UV1). These devices are 2K x 4K, backside illuminated CCDs with 15 micron
pixels, which are representative of the technology to be implemented in the WFC3
flight devices.

Both devices were characterized prior to irradiation and performed well; the
readout noise, CTE and dark current all exceeded project specifications. The
devices were irradiated at the UC Davis synchrotron with 1 x 109 cm-2
of 63 MeV protons which is expected to produce the equivalent of 1 year of
on-orbit proton damage. The post-radiation results showed normal deterioration
of the CTE in both devices, but the readout noise did not change, and dark
current was again well within project requirements.

Dark Current

A sequence of dark exposures of 1, 2, and 4 hour duration were taken at three
different temperatures: -80C, -90C, and -100C. Preliminary results show that the
pre- and post-radiation mean dark currents do not exceed the project
specification of less than 20 electrons/pixel/hour. The proton irradiation
causes defects in the silicon lattice. These defects are responsible for
excessive dark current generation in some pixels (warm and hot pixels).
Figure 1 and
Table 1
show the expected increase in the tail of dark current distribution
from pre- to post-irradiation.

Each device was exposed to an 55Fe source for a series of exposure
times from a low of 0.5 seconds to a high of 100 seconds. By varying the
exposure time to the 55Fe source we can vary the amount of time
between the clocking out of x-ray events (delta time), the goal being to gain
information on the characteristic timescales associated with the charge traps.
This suite of images was taken at -80C, -90C and -100C. The array was read with
two amplifiers simultaneously which effectively splits the array into two parts:
channels A and B.Sample histograms of pre- and post-radiation signal intensities from 5 second
images are shown in Figure 2.
The K-alpha line is seen at about 6000 ADU and
K-beta line at about 6800 ADU in the pre-radiation (blue) data. The mean value
(or center of the peak) of K-alpha is used to compute the gain. The
post-radiation (red) histogram shows that K-alpha has broadened out and that
K-beta is not resolved.

Figure 3
is an example of a stacking plot of post-irradiated 55Fe
data, with the upper and lower bounds of the K-alpha band, and the linear best
fit to that area. Using this method of obtaining the slope of the best fit line
and dividing it by the number of electrons/photon (1620 for 55Fe) is
the primary method used to calculate CTE from the 55Fe images.

Figure 4
is a plot of parallel Charge Transfer Inefficiency (CTI) vs Delta
Time of the pre-irradiated CCD44UV1 at -80C, -90C, and -100C. Points
corresponding to a delta time of ~100 seconds represent ~1 event/column or the
most sparse event density practical.
Figure 5
shows a similar plot for the
irradiated CCD. Note that both figures show an exponential increase in CTI as
the event density decreases. This is because at high event densities,
corresponding to less time between the clocking out of events, some traps are
filled by preceding events, resulting in better charge transfer for trailing
events. Note also the marked decrease in CTI at lower temperatures after the
radiation damage has occurred. It is preferable to operate a damaged device at
colder temperatures, at least over the range of temperatures explored here.

The readout noise was measured to be a very low 2.1-2.3 e- rms for
both channels using two different sources: a bias corrected, 10 second dark
image, and the overscan area of several image files. The read noise in ADU is
given by the width of a Gaussian fit to the pixel intensities. The gain is
applied to convert to e- rms.

Conclusions

1. The two engineering grade Marconi devices performed extremely well and
behave similarly, both pre- and post-irradiation.

2. Project specifications are met with 1) read noise of 2.2 ± 0.1 e-
rms, 2) a pre-radiation parallel CTE of 0.999997±1.0x10-6, and a
pre-radiation serial CTE of 0.999998±1.0x10-6, a post-radiation
parallel CTE of 0.999951±5.0x10-6, and a post-radiation serial CTE
of 0.999987±5.0x10-6 (numbers are an average for both devices at -90
C), and 3) dark current below 20 electrons/pixel/hour for both devices in pre-
and post-radiation data.

3. Both devices exhibited a marked temperature dependency for CTI in the
post-radiation data, as well as a trend towards higher CTI for sparser photon
event density in both pre- and post-radiation data.

Future Work

Further work is in progress on CTE measurements. Extended Pixel Edge Response
(EPER) and First Pixel Response (FPR) measurements are in progress in order to
extend our understanding of the performance of radiation damaged devices, with
the ultimate goal of being able to predict the performance of the WFC3 detectors
over the instrument's lifetime. Further experimentation will also allow an
investigation of methods for mitigating the degradation in CTE performance and
in making appropriate corrections to real photometric data.